1. Field of the Invention
The present invention relates to a charged particle beam writing apparatus and an optical axis deviation correcting method for a charged particle beam, for example, correction for optical axis deviation of an electron beam in a writing apparatus which forms a pattern on a target object while variably shaping the electron beam.
2. Related Art
A lithography technique which takes a part of the development of miniaturization of semiconductor devices is only a process, in which a pattern is generated, in semiconductor manufacturing processes and is very important. In recent years, with the advancement in integration density of an LSI, circuit line widths required for semiconductor devices are miniaturized year by year. In order to form desired circuit patterns on the semiconductor devices, precise original patterns (to be also referred to as a reticle or a mask) are required. In this case, an electron beam writing technique has an essentially excellent resolution, and is used in production of precise original patterns.
FIG. 8 is a conceptual diagram for explaining an operation of a variable-shaped electron beam writing apparatus. The variable-shaped electron beam (EB) writing apparatus operates as follows. An oblong, for example, rectangular opening 411 to shape an electron beam 330 is formed in a first aperture plate 410. A variable-shaped opening 421 to shape the electron beam 330 having passed through the opening 411 of the first aperture plate 410 into a desired oblong shape is formed in a second aperture plate 420. The electron beam 330 irradiated from the charged particle source 430 and having passed through the opening 411 of the first aperture plate 410 is deflected by a deflector, passes through a part of the variable-shaped opening 421 of the second aperture plate 420, and is irradiated on a target object 340 placed on a stage continuously moving in one predetermined direction (for example, an X direction). More specifically, an oblong shape which can pass through both the opening 411 of the first aperture plate 410 and the variable-shaped opening 421 of the second aperture plate 420 is written in a write region of the target object 340 placed on the stage continuously moving in the X direction. A scheme which causes an electron beam to pass through both the opening 411 of the first aperture plate 410 and the variable-shaped opening 421 of the second aperture plate 420 to form an arbitrary shape is called a variable-shaping scheme.
In this case, in the electron beam writing apparatus, an operation of irradiating a beam on a target object and an operation of cutting a beam not to irradiate a beam are performed. In the operation of irradiating a beam on a target object, the electron beam is caused to pass through an opening of a blanking aperture plate. In the operation of cutting a beam not to irradiate the beam, an electron beam is deflected by a blanker to a blocking portion of the blanking aperture plate to block the electron beam. These operations are repeatedly performed to form a pattern on a target object. Since the number of figures of a formed pattern increases with the miniaturization of semiconductor devices, a blanking mechanism which can perform the operations at a high speed is required to realize high productivity. In order to realize a high-speed operation, a method which connects terminal resistors to electrodes of the blanker to match impedances and suppresses a reflected wave of a blanking signal in application of a voltage is known (for example, see Published Unexamined Japanese Patent Application No. 11-150055).
Before a pattern is formed, an optical axis of an electron beam must be adjusted. However, in a conventional technique, the optical axis of the electron beam is adjusted such that a voltage applied to a blanker is set to a certain stationary state (in this case, 0 V) to make a beam-ON state so as to cause an electron beam to pass through a center of an opening of a blanking aperture plate. When a high-frequency pulse serving as a blanking signal to control ON/OFF state of an electron beam is applied across electrodes of the blanker having the above configuration by using the writing apparatus having the optical axis adjusted as described above, the optical axis of the electron beam which should have been adjusted may be disadvantageously deviated. This phenomenon may be caused by the following factor. That is, for example, when terminal resistors are attached to blanking electrodes to suppress a reflected wave, a current is concentrated on a surface of each of the electrodes by a skin effect at the corresponding electrode when a high-frequency pulse is applied across the electrodes. For this reason, at a moment an applied voltage of the high-frequency pulse is made zero, a residual current having flowed on the surfaces of the electrodes generates an eddy current to generate a magnetic field.
When the optical axis of the electron beam is deviated, even though the beam is turned on to make a state in which the electron beam can pass through the blanking aperture plate, the electron beam is partially cut by the blanking aperture plate to decrease a current transmittance. For this reason, a dose of an electron beam reaching a target object may decrease. The decrease in dose of the electron beam causes a problem such as deterioration of pattern dimensional accuracy.
Furthermore, when the optical axis of the electron beam is deviated, the electron beam does not pass through the center of an electron lens such as an objective lens through which the electron beam were to pass thereafter to also cause an on-axis astigmatism, a deflection astigmatism, and a deflection distortion.
As described above, when a high-frequency pulse is applied across the electrodes of the blanker having the above configuration, the optical axis of the electron beam which should have been adjusted is disadvantageously deviated. According to an experiment by the inventors, with respect to this deviation, an amount of deviation becomes notable according to an irradiation cycle ratio. As described above, when the optical axis of the electron beam is deviated, a problem such as deterioration in pattern dimensional accuracy, an on-axis astigmatism, a deflection astigmatism, or a deflection distortion may be caused. However, conventionally, a method of solving the problem has not been established yet.